This invention relates to an image signal processor, and more particularly relates to the image signal processor capable of performing image transform processing over a local portion which is specified in the encoded image signals spatially or on a time series basis.
Generally, the image transform processing (for example, editing, conversion, level position transform) of the image signals encoded by means of the high efficiency encoding, is carried out as follows. That is, the encoded image signals are decoded once at first. In the next, the image transform processing is performed based on these decoded image signals. Finally, the transformed image signals are again encoded.
For instance, in the case of composing two image signals which have been encoded according to the Discrete Cosine Transform (referred to as DCT hereinafter), the image transform processing of these image signals can not be carried out before they are decoded by the inverse DCT. An example of the processing for composing the image signals encoded by DCT will be described with reference to FIG. 2. This figure is a block diagram of a prior art image signal processor which performs non-linear image editing with the use of a hard disk memory.
In the figure, reference numeral 31 designates a hard disk memory, in which there are stored the encoded image signals which have been encoded according to such an encoding standard as the Joint Photographic Coding Experts Group (JPEG) or the like. The encoded signals are read out from the disk memory and decoded by a decoding circuit 32, thereby decoding them back to the original component signals (the image signals are treated in the form of the component signals in JPEG). A processing circuit 33 transforms these component signals by way of cutting, inserting, and/or composing the image signals. The image signals (component signals in this case) finished through the image transform processing are again encoded by means of an encoding circuit 34 and stored in the hard disk memory 31. Referring to FIG. 2 again, reference numeral 35 designates a key box, which serves as a source for generating control signals instructing the processing circuit 33 to perform the image transform processing. Upon receipt of the control signals, the processing circuit 33 executes the image transform processing.
Now, in case of encoding the image signals by means of the DCT or the like in order to enhance the encoding efficiency, it is often observed that a considerable amount of the image information is dropped in the course of encoding the image signals. Therefore, if the encoded image signals are decoded as they are, it occurs that the difference between the decoded image signals and the original ones is made larger. In this way, as the encoded image signals are irreversible signals, the image quality is made worse at every repetition of encoding and decoding of image signals.
However, according to the prior art image signal processor as described above, in the case of editing the encoded image signals, it is required that decoding and encoding steps have to be performed over the entirety of the encoded image signals. This encounters the problem that repetition of the image transform processing, for instance editing of the image, leads to the severe degeneration in the quality of image signals.
Accordingly, an object of the invention is to provide a novel and improved image signal processor, in which degeneration in the quality of image signals can be suppressed to the minimum even though encoding and decoding of image signals are repeatedly executed for the image transform processing.
Furthermore, another object of the invention is to provide a novel and improved image signal processor, in which the image transform processing can be carried out only over the necessary minimum unit of the image signals which is specified spatially and/or on a time series basis, thereby shortening the time required for the image transform processing and at the same time, enhancing the efficiency of the image transform processing.
These and other objects of the invention will be made more clear by the description as is made in the following.